U.S. Pat. No. 4,884,004, issued on Nov. 28, 1989 to Deal et al., discloses a heat dissipative, electron reflective coating which is applied to a surface of the color selection electrode or shadow mask of a color CRT. The coating comprises a compound selected from the group consisting of bismuth (tri)oxide-potassium silicate and tungsten-potassium silicate.
The color CRT comprises an evacuated envelope with a multibeam electron gun at one end and a cathodoluminescent screen at the other end. A shadow mask is located adjacent to the screen. During operation of the CRT, only a small percentage of each electron beam passes through apertures in the shadow mask and impinges upon the screen. At about the center of the shadow mask, the masking plate intercepts all but about 18% of the beam; that is, the shadow mask is said to have a transmission of about 18%. The electrons comprising the remaining 82% of the beam are intercepted by the mask plate on their way to the screen. For shadow masks not having the coating described in U.S. Pat. No. 4,884,004, the kinetic energy of the intercepted electrons is converted to thermal energy with an increase in shadow mask temperatures resulting in thermal expansion of the mask. Since the shadow mask is usually supported by a frame of substantial mass, the temperature of the mask during initial warm-up will rise more rapidly in the center than at the edge. This causes the mask to dome, so that the center portion of the mask moves toward the screen, while the edge of the mask maintains its spacing with the screen. Furthermore, when a large number of electrons impinge upon a local area of the mask, to create high picture brightness, localized doming or blister warpage occurs unless temperature equilibrium in the plane of the mask is reestablished sufficiently rapidly. Both blister warpage and overall doming of the mask result in color errors due to electron beam misregister with the phosphor elements of the screen. The coating materials described in U.S. Pat. No. 4,884,004 are effective in reducing doming and blister warpage when the coating is applied to the electron gun-facing surface of the shadow mask.
Since the cost of a shadow mask assembly, i.e., a shadow mask and frame, increases with increasing CRT size, it is desirable to have a method of salvaging the coated shadow mask assembly by removing a defective or otherwise unacceptable coating therefrom.